Measuring devices, especially analytical devices, such as turbidity sensors for examining aqueous media or other samples, usually include at least a first sensor for a measured variable. The signal of such sensor is mapped according to a mapping specification to an output, measured value. The mapping specification can comprise, for example, a constant factor, a linear equation, a polynomial, or some other expression. Besides depending on the signal of the first sensor, the mapping specification can also depend on signals of at least a second sensor, in order, to compensate, for example, cross-sensitivity of the first sensor to the measured variable of the second sensor, for example temperature. The mapping specification is representable, for example, in the form of characteristic curves. The parameters of the mapping specification, for example coefficients and zero-point offset, are usually stored in a data memory of the measuring device.
Ordinarily, a measuring device is calibrated at the point in time when it is put in service, so that the stored parameters of the mapping specification agree with the required parameters, to an extent such that the output measured values are the same as the actual values of the measured variable, within the desired accuracy of measurement.
Measuring devices are subject, however, to drift—i.e., due to wear, aging, fouling or other causes, the output measured value changes from the actual value of the measured variable. In this case, then the stored parameters of the mapping specification are no longer up-to-date and must be made current by a re-calibration.
For pH measuring devices, this is done, for example, by performing measurements at two different pH values with two standard liquids and by so adjusting zero-point and slope of the measuring device on the basis of the output measured values, that the measuring device outputs the correct measured values for the reference liquids.
This manner of proceeding cannot be used, for example, for fixedly installed, turbidity measuring devices for the monitoring of drinking water, since it is not practical to take these out of the drinking water and subject them to standard samples of defined degree of turbidity.
Instead of this, these turbidity measuring devices are calibrated in the so-called laboratory reference method. For this, the current measured value at the point in time of a calibration measurement is stored as calibration measured value. Simultaneously, a sample of the medium is taken and a laboratory reference measurement is performed in the laboratory on the taken sample. The result of the laboratory reference measurement is used as laboratory reference measured value for calibrating the measuring device.
From the difference between calibration measured value and laboratory reference measured value, one obtains the current deviation of the measuring device at the laboratory reference measured value. Lacking further information, this deviation is usually associated with a zero point displacement and the corresponding parameter of the mapping specification of the measuring device is updated. This manner of proceeding is problematic to the extent that the behavior of a measuring device can also change in a manner such that it would require the correction of other parameters of the mapping specification, for example slope in the case of a linear mapping specification.
It is, therefore, an object of the present invention to provide a calibration method for measuring devices for overcoming the described disadvantages.